The recognition structures of T cells have been examined using anti-receptor antibodies and anti-T cell surface antigen specific antibodies prepared against cytotoxic T cell clones. Antisera produced in mice and rats against the CTL clones show specific reactivity and immunoprecipitation of receptor molecules on the cytotoxic T cells as well as other molecules which appear to be critical to T cell recognition. Present studies are designed to look at monoclonal antibodies prepared against the T cell receptor and other cell surface antigens. Three such monoclonal antibodies have been developed which specifically inhibit T cell function, precipitate glycoprotiens of varying molecular weights, and are present on activated cytotoxic T cells but not precursor resting T cell populations. The antibodies generated are IgM, k. One antibody, 120-3-7-2, may recognize molecule similar to the T200 molecule while antibodies 120-4-24 and 128-12-17 recognize unique structures on the T cell surface which have not previously been described. In addition, a series of other monoclonal antibodies which recognize an Lyt6-linked molecule on the surface of the cytotoxic T cell clones has been generated. These monoclonal antibodies appear to subdivide class I specific CTL into two distinct populations. Current studies are underway to develop monoclonal antibodies against constant region portions of the T cell receptor as well as variable region determinants. In addition, further studies will be designed to analyze the monoclonal antibodies which have been developed and to better define the structural determinants involved in T cell function which are expressed on cytotoxic T cells.